[cypher]

Don't give up too easily, make sure you wash your hands really well with an unscented soap before having another go just in case she can smell anything which might cause her to nip.
Yanking your hand away will have scared her but she'll get over it, don't rush it just wait a while & have another go but bury your thumb in the substrate too!
You could try gloves, I've never used them, it may stop her nipping but she won't get used to your hands that way.
If she's used to your hands always offering food she may just be looking for more so just keep trying if you can.